1. Field of the Invention
The present invention generally relates to drilling and, in particular, to performing, in a downhole location, modal analysis of a borehole.
2. Description of the Related Art
Boreholes are drilled deep into the earth for many applications such as carbon dioxide sequestration, geothermal production, and hydrocarbon exploration and production. In all of the applications, the boreholes are drilled such that they pass through or allow access to a material (e.g., a gas or fluid) contained in a formation located below the earth's surface different types of tools and instruments may be disposed in the boreholes to perform various tasks and measurements.
In more detail, wellbores or boreholes for producing hydrocarbons (such as oil and gas) are drilled using a drill string that includes a tubing made up of jointed tubulars or a continuous coiled tubing that has a drilling assembly, also referred to as the bottom hole assembly (BHA), attached to its bottom end. The BHA typically includes a number of sensors, formation evaluation tools, and directional drilling tools. A drill bit attached to the BHA is rotated with a drilling motor in the BHA and/or by rotating the drill string to drill the wellbore. While drilling, the sensors can determine several attributes about the motion and orientation of the BHA that can used, for example, to determine how the drill string will progress. Further, such information can be used to detect or prevent operation of the drill string in condition that is less than favorable. One example that is considered is vibrations that may be created in the drill string.
Severe vibrations in drill strings and associated bottomhole assemblies can be caused by cutting forces at the bit or mass imbalances in downhole tools such as mud motors. Vibrations can be differentiated into axial, torsional and lateral direction. Negative effects due to the severe vibrations are among others reduced rate of penetration, reduced quality of measurements and downhole failures. Hence, improvements in drill string operations that prevent severe vibrations would be appreciated in the drilling industry.